Transactivation experiments were performed involving the genetically related Salmonella phages P22, L and Px1 in order to find out if more than one positively acting regulatory product is engaged in the expression of vegetative gene functions of each of these phages. The results obtained with Px1- and L-lysogenic cells superinfected with P22 suggest the following conclusions: 1. The expression of the early genes 12 and 23 and of the late gene 19 (lysozyme synthesis) is positively regulated by two different regulatory products, since P22 transactivates in prophage Px1 both early and late genes (Prell, 1973), in prophage L only late genes. 2. The transactivation by P22 of the lysozyme gene of prophage L takes place in the presence of L repressor. This conclusion is suggested, since the superinfecting P22 does not derepress early gene expression (see 1.), and is confirmed by demonstration of replication inhibition for L phage in L lysogenic cells doubly superinfected with L and P22 phages (Thomas-Bertani-experiment). 3. The late gene regulatory protein seems to be synthesized by gene 23, as transactivation experiments with both L- and Px1 prophages suggest. 4. The expression of gene 23 itself is turned on by an early regulatory product. The gene which codes for it is still unidentified. However its product seems to by highly specific, since it is active on Px1- but not on L-prophage.